Telephone exchanges of the direct-dialing type have multilevel rotary line selectors which, in contrast to marker-controlled selectors as described in commonly owned U.S. Pat. No. 3,634,629, have wipers that are directly stepped by replicated dialing pulses so as to come to rest on bank contacts connected to the desired outgoing lines. High-speed selectors of the noble-metal type, whose wipers may sweep their contact banks at a rate of 170 steps per second, for example, are generally provided on each level with a multiplicity of bank contacts divided into decadic groups that are separated by intervening rest positions on which a test wiper may come to a halt after selection of one of these decadic groups by the penultimate digit of a call number. Starting from this rest position, the wiper is further advanced by one to ten steps within the preselected decade by the final call-number digit. For a more detailed description of this well-known type of rotary selector, reference may be made to a book by Fulvio Vallese, entitled "Elementi di Commutazione Telefonica" and published by Edizioni Scientifiche Siderea, Rome 1966, pages 133-144.
Such rotary selectors (designated SMN in the above-identified Italian publication) can also be used in selector stages preceding the line selector to find an available selector of the next-following stage. In that instance the test wiper is again stepped from one rest position to the next, over the intervening bank contacts which are preferably but not necessarily ten in number, by the dial pulses of a call-number digit and is then advanced in a free-search mode across the next group of intervening bank contacts until it finds a free test wire or, if all interstage connections are occupied, until it comes to a stop on the last bank contact of that group with emission of a busy signal. The stepping of the selector motor in that mode is controlled by several relays including a test relay which, on being connected to the test wiper, detects a free line and deactivates a previously operated driving relay, the free state of the line being indicated by the closure of switch contacts which are opened when the line is busy. Such a free-search mode of selection has also been used heretofore in a line selector for finding a free line in a multiple connected to consecutive bank contacts.